Supported noble metal catalysts, in which relatively small noble metal particles are deposited on the surface of a solid support, are used in particular in synthetic chemical and petrochemical processes in order to convert a wide variety of educts into desired intermediate products or end products or to chemically refine different cuts of petroleum processing. In addition, supported noble metal catalysts are in particular also used as oxidation catalysts in the purification of exhaust gases from combustion engines.
Supported catalysts loaded with noble metal are normally produced by means of a multi-stage method. For example, in a first step a support material is impregnated with a noble metal salt solution of the desired noble metal. After the removal of the solvent from the support material in a subsequent step, the support material is then calcined in a further step, wherein the noble metal can be converted to an oxide form by thermal treatment. Then, in a further step, the noble metal component is converted to the catalytically active, highly dispersed noble metal of oxidation state 0, for example by means of hydrogen, carbon monoxide or wet-chemical reducing agent. The supported noble metal catalyst can be stabilized for storage purposes in a final step, for example by wet stabilization by means of an oil or by dry stabilization by means of preoxidation (passivation) of the deposited noble metal particles.
The activity of supported noble metal catalysts normally depends on the size of the noble metal particles. The supported noble metal catalysts known in the state of the art have the disadvantage that they become less active in the course of their use because of a sintering of the noble metal particles into larger units accompanied by a reduction in catalytically active surface. The speed of this so-called thermal ageing process depends on the temperature level at which the catalyst is used. To be precise, as the operating temperature increases so does the speed of said ageing process, which is assumed to be caused by an increased mobility of the noble metal particles on the support material surface accompanied by an increased tendency to sinter.
A large number of attempts have already been made in the state of the art to produce catalysts which have a high activity when used at high temperatures and are subject to only a low thermal ageing process. Kubanek et al., “Microporous and Mesoporous Materials 77 (2005) 89-96”, for example describe the production of a supported platinum catalyst by impregnating a zeolite of the structure type MFI (SH27) with the Pt precursor compound Pt(NH3)4(NO3)2 and then calcining the zeolite loaded with the precursor compound in a protective gas atmosphere. When Pt(NH3)4(NO3)2 is used, autoreduction occurs at relatively high temperatures. However, the thus-produced supported platinum catalyst has a relatively low activity as well as a relatively high tendency to thermal ageing.